<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
   <script>
       var num1 =parseFloat(prompt("请输入第一个数"))
       var fg =prompt("请输入运算符")
       var num2 =parseFloat(prompt("请输入第二个数"))  
     if(fg=="+"){
       alert(num1+num2)
       }else if(fg=="-"){
       alert(num1-num2)
       }
       else if(fg=="*"){
       alert(num1*num2)
       }
       else if(fg=="/"){
       alert(num1/num2)
       }else{
           alert("请输入正确数字")
       }

    
   </script> 
</body>
</html>